The present invention relates to a sucker rod for a well pump and to a method and apparatus for attaching an end fitting to a sucker rod.
During production of a well, such as an oil well, the pressure from the reservoir may become insufficient to force the fluid to the surface. If so, a pump attached to the end of a sucker rod string may be lowered into the well. The upper end of the sucker rod string is typically attached to a pump jack or similar apparatus on the surface. The pump jack reciprocates the sucker rod string to alternately raise and lower a piston in the barrel of the pump which is submerged in the fluid in the well. When the piston is raised in the barrel by the sucker rod string, a check valve in the piston closes preventing fluid above piston from flowing back into the barrel and the lowering pressure in the barrel opens a check valve in the barrel allowing fluid in the well to flow into the barrel. When the piston is lowered in the barrel by reciprocation of the sucker rod string, the check valve in the barrel closes trapping fluid in the barrel and the check valve in the piston opens enabling the piston to move downward in the barrel causing fluid in the barrel to flow past the piston where it will be trapped when the piston is raised on the next upstroke thereby raising the level of fluid in the well.
Steel sucker rods, typically about twenty five (25) feet in length and threaded at each end, have been joined end-to-end to make up sucker rod strings. However, steel is heavy and powerful equipment is required to reciprocate a steel sucker rod string. In addition, steel is subject to corrosion in the environment of a well and repair or replacement of failed steel sucker rod strings is expensive and difficult.
Fiberglass sucker rods were introduced in the 1970's. A fiberglass sucker rod comprises a fiberglass rod and an end fitting affixed to each end of the rod. Fiberglass sucker rods are typically 37.5 feet in length although 25 and 30 foot lengths and custom lengths are available. A fiberglass sucker rod weighs approximately one-third of the weight of an equal sized steel sucker rod making transportation, handling and installation significantly easier and less expensive and reducing the cost of the pump jack and the power necessary to reciprocate the sucker rod string.
The fiberglass rod, commonly available in diameters ranging from 0.625 inches to 1.25 inches, comprises long parallel strands of glass fiber in a plastic matrix. The fiberglass rod is typically formed by the pultrusion process where glass fiber is fed through a carding plate and then impregnated with a thermosetting resin such as vinyl ester, isothalic polyester or epoxy and preheated with a radio frequency preheater. The impregnated fiber is then pulled through a heated die which forms the final shape and size of the rod and cures the thermosetting resin.
The end fittings of a fiberglass sucker rod are typically made of steel and have external shapes and dimensions conforming to recommendations of the American Petroleum Institute (API). A cylindrical first portion of the elongate end fitting extending longitudinally from a first end of the fitting includes a surface (called a coupler) defining a screw thread enabling joining of the sucker rod to another sucker rod when making up a sucker rod string. The two end fittings of a sucker rod may have threads of opposite gender enabling an end fitting of one rod to be threaded directly into an end fitting of a second sucker rod or the end fittings may be the same gender requiring a coupling having threads of the opposite gender to join the end fittings of the sucker rods.
A second longitudinal portion extending from the first or the threaded portion of the end fitting toward the second end of the fitting defines a square cross-section providing plural flat surfaces for engagement by a wrench enabling the application of torque to the fitting when making up the sucker rod string.
A third longitudinal portion extending from the second portion to the second end of the fitting typically has an annular cross-section with a cylindrical outer surface and an inner surface defining a rod cavity extending longitudinally in the end fitting from an aperture in the surface of the second end of the fitting. The rod cavity is typically circular in cross-section with a diameter which varies along the longitudinal axis of the fitting to define one or more substantially frustoconical cavity portions arrayed end-to-end along the longitudinal axis of the fitting with the larger diameter of the frustrum most remote from the second end of the fitting.
Typically, the end fittings are attached to the fiberglass rod with a thermosetting adhesive which adheres to the fiberglass rod and which hardens to form a wedge(s) in the frustoconical portion(s) of the rod cavity. To prevent the adhesive from adhering to the steel end fitting, the surface of the rod cavity is coated with a release agent which is cured. The adhesive resin, such as epoxy, is added to the rod cavity and the fiberglass rod is inserted into the cavity. Typically, the resin is cured by heating the sucker rod assembly for approximately one hour. After the adhesive resin has cured, tension is applied to the rod to set the adhesive wedges in the steel fitting. Since the adhesive resin is not adhered to the end fitting, the fitting is restrained to the rod by the bond between the adhesive and the rod and the physical interference between the wedge(s) of cured adhesive and the corresponding frustoconical surface portion(s) of the rod cavity.
As the sucker rod string is reciprocated, cyclic tension and other forces may be exerted on the sucker rod. As a result of the angular orientation of the conical surface of the adhesive wedge to the longitudinal axis of the fiberglass rod, a component of the tension force on the sucker rod is exerted normal to the longitudinal axis of the fiberglass rod radially compressing the rod. In addition, the cyclic nature of the forces exerted on the sucker rod is believed to cause creep in the adhesive wedge further radially compressing the fiberglass rod near the intersection of the rod and the smaller diameter end of the conical wedge. Although the exact nature of the failure mode is unknown, fiberglass sucker rods commonly fail proximate the point at which the rod meets the end fitting where the radial compression of the rod is expected to be greatest. Since introduction of fiberglass sucker rods there have been continued efforts to improve the sucker rod with much of the effort directed to the relationship of the steel end fittings and the fiberglass rod and in particular to changing the shape of the interface between the cured adhesive, the fiberglass rod and the end fitting.
What is desired, therefore, is a fiberglass sucker rod assembly which is stronger, has improved life and more consistent performance.